There was an old thread made for discussion on where to buy the Japanese copy, here's a quick link to it [
Where to buy]. Be cautious it is over a month old, so reviving it would only get it closed.
Though, a simple Google search would have also given you a few other sources on where to purchase the Japanese copy of HeartGold and SoulSilver. I would have recommended Play-Asia, but it seems you've already pointed it out. I just say to be smart on where you purchase the game, because a more expensive copy coming from a more reliable site would be better than getting a bogus copy of the game any day.